Among a multitude of different battery types, there exist so-called thin film batteries. Those batteries only comprise a thickness in the range of nanometers, micrometers or millimeters. They may exhibit comparatively small dimensions and are therefore generally applicable to a large spectrum of different applications. Generally, such batteries or electrochemical cells can be formed into any arbitrary shape. They can be stacked, used in parallel and generally provide a comparatively large energy density.
Thin film electrochemical cells may also provide a particular mechanical flexibility. Hence, they are typically bendable and elastically deformable to a certain degree.
Electrochemical cells typically comprise an anode, an anode current collector, a cathode, a cathode current collector and a separator extending between anode and cathode. The electrochemical cell is impregnated with electrolyte and since it is generally liquid, it must be contained inside a tight external encapsulation. Moreover and in order to provide a desired mechanical flexibility, anode and cathode current collectors have to provide corresponding flexible properties. Since the current collectors are typically arranged at the outside facing portions of anode and cathode, they may become particularly subject to mechanical stress when the electrochemical cell is bent.
In practical applications and in particular after a number of bending or folding operations, e.g. after about 100 or 500 bending operations on a radius of curvature of 10 mm, conventional current collectors may exhibit a fissured structure or may even tend to show damaged portions at least on their surface. Such current collectors typically formed on the basis of metal foils therefore tend to deteriorate and leak in response to repeated bending or folding operations.
A multilayer structure of a thin and flexible electrochemical cell, such like a film-type battery is rather sensitive to bending or folding when the distance between mutually bonded or interconnected layers is rather large. When for instance two overlapping substrates 1, 2 as illustrated in FIG. 1 are mutually bonded along a lateral edge, e.g. by means of an adhesive 3 featuring a particular thickness, a bending of the substrates 1, 2 into a curved or bended configuration 1′, 2′ leads to a substantive shearing of the adhesive 3′. In order to bond or to interconnect substantially overlapping layers of e.g. an electrochemical cell, either a rather flexible adhesive has to be used or the thickness of the adhesive should be reduced to a minimum, in order to reduce such a bending-induced shearing and its negative impact on the tightness and durability of the adhesive.
On the one hand, rather flexible or elastic deformable adhesives may not provide sufficient gas barrier properties so that a multilayer structure or a respective electrochemical cell may exhibit overly fast aging. On the other hand, making use of a comparatively rigid or stiff adhesive featuring a sufficient gas tightness for the layers to be mutually bonded should not exceed a maximum thickness in order to enable a flexible bending of the multilayer structure at all.
However, when mutually bonding two current collectors in a battery application, serving as anode current collector and as cathode current collectors, their distance with respect to each other cannot be reduced below a predefined minimum of e.g. 400 μm. Otherwise, a desired electrical capacity of the battery cannot be achieved due to a reduced volume for the electro-active material.
It is therefore an object of the present invention to provide an improved electrochemical cell exhibiting a well defined and large degree of mechanical flexibility and tolerating repeated bending or folding operations with comparatively small bending radii. Additionally, the internal structure of the electrochemical cell should be rather simple, space saving and its production should be cost efficient. Moreover, the electrochemical cell should exhibit a multi-dimensional folding or a multi-dimensional flexibility, i.e., the electrochemical cell should be bendable with regard to at least two different directions.